MicroRNAs (miRNAs) form an extensive class of RNA molecules that regulate gene expression at post-transcriptional level. In recent years, much progress has been made in dissection of biogenesis and functions of miRNAs. There are at least several hundred miRNA genes in the human genome, and the emerging evidence suggests that miRNAs are broadly implicated in gene regulation. Here, we review some recent advances, and particularly we discuss how comparative genomics helps to identify novel miRNA genes, how studies in zebrafish reveal roles of miRNAs in morphogenesis, how changes in miRNA expression patterns are connected with cancer and how host–virus coevolution exploits miRNA regulatory pathways.